One objective of this project is the elucidation of the mechanisms underlying light adaptation, a process in which retinal response properties adjust to variations in ambient illumination. The effects of light adaptation on the sensitivity and dynamics of visual responses of neurons in the turtle retina will be studied. This will involve recording intracellularly the temporal frequency responses of cones and horizontal cells (H-cells) to light that is modulated sinusoidally around various mean levels. One hypothesis to be tested is that changes in the sensitivity and dynamics of cone responses, as the mean light level is varied, can be accounted for by a simple phenomenological feedback model in which the strength of a feedback signal is adjusted to be proportional to the mean light level. This model has already been shown to account for the linear component of H-cell responses to modulation of light around a wide range of mean levels. There are two candidates for this alleged feedback signal: neural feedback from H-cells to cones and chemical feedback within the cone phototransduction mechanism. The role of neural feedback will be studied by controlling in various ways both the steady and the transient component of H-cell feedback, while monitoring the effects on cone adaptation. Another hypothesis to be tested is that a somewhat more elaborate feedback model, in which the dynamics of adaptation are included, can account for both linear and nonlinear components of cone and H-cell responses over a wide range of light levels. In light of recent progress in unraveling the molecular basis of phototransduction, an attempt will be made to make a connection between the phenomenological feedback model and biochemical mechanisms. The goal is to develop biochemical kinetic models for cone phototransduction that are consistent with the known biochemistry and also with the electrophysiological behavior of cones. The feedback model with its highly distinctive mathematical form will provide a powerful tool for testing such models. The effects of light adaptation on the receptive field properties of cones will also be studied; cone spatial frequency responses to drifting sinusoidal gratings superimposed on various background light levels will be measured.